1. Field of the Invention
This invention relates to the field of electronic packaging, and more patricularly pertains to the process of fixing metal pins to a brittle substrate.
2. Description of the Prior Art
Integrated microelectronic circuit chips are frequently mounted on ceramic substrates that provide planar surfaces on which electrical lines are formed for interconnecting the input and output terminals of the chips and other portions of the electronic system of which they are a part. Connections are made between the various circuits with which the chips interface by copper pins, which are electrically connected to the terminals of the chips and are mechanically attached and held in position after being fixed to the ceramic substrate. Unfortunately, ceramics are brittle, frangible materials that are easily shattered when large forces, particularly abrupt tension forces, are applied in the process of fixing the pins to the substrate. In the process of joining the pin and substrate, large tensile forces necessarily result in the vicinity of the holes that are formed through the ceramic into which the pins are inserted. Therefore, it is conventional practice in the process of fixing the connecting pins to the ceramic substrate that the force which attaches the pin and forms the pin head be slowly applied and of minimum magnitude so that damage to the ceramic is avoided.
U.S. Pat. No. 3,484,937 describes a method for connecting a pin to a printed circuit board in which a hole is drilled through the thickness of the board, made of fiberglass impregnated cloth having a high tension strength and one particularly tolerant of impact force applied in the process of pinning. The pin is inserted in a hole whose diameter is slightly larger than the diameter of the pin. The ends of the pin blank are restrained to limit radial expansion while the pin is longitudinally compressed. This causes an expansion of its center portion radially outward into engagement with the surface of the hole. The extent of the compression and radial expansion of the pin depends on the particular board material being used and is such that the mechanical connection has strength adequate to assure proper retention of the pin in the hole.
Ceramic substrates are rigid and brittle materials. They fail easily by cracking along lines that run between pin holes drilled through the thickness of the material when high forces are applied to the copper pin the the process of fixing the pins to the printed circuit board. A method for fixing pins in a substrate whose holes are somewhat smaller than the pins so as to establish an interference or press fit between the pins and the holes is described in U.S. Pat. No. 3,566,464. This system is not suitable for pinning substrates of ceramic material because the end of the pin that pushes against the rim of the substrate surrounding the small diameter hole produces a force that can readily crack and damage a ceramic substrate.
Other pinning systems, particularly those in which the pins are driven at high speed to form an opening either partially or completely through the body of the substrate, are unsuitable for pinning ceramic substrates because the striking force and propelled pins produce tension forces which can easily damage or fail the body of the ceramic substrate. A high velocity pinning system of this kind is described in Canadian Pat. No. 891,180 and in the publication by H. P. Byrne, Terminal pin projector, IBM Technical Disclosure Bulletin, volume 9, number 4 (1966) page 365.
A conventional method, known as the head and bulge system, is frequently used to pin ceramic substrates. According to this method, a pin blank is inserted in a larger diameter hole formed in the substrate so that the pin extends outward on each side of the substrate. A head is formed on one side of the substrate and a bulge is formed on the pin at the opposite end. The substrate material that surrounds the hole located between the head and the bulge becomes tightly wedged between them and in this way secures the pin to the substrate. Examples of systems that employ the head and bulge method of securing pins to the substrate are described in U.S. Pat. Nos. 3,216,097; 3,257,708 and 3,768,134. These systems are not satisfactory for fixing pins to a ceramic substrate because the slowly applied force required to form the bulge and head is so large that it often causes the ceramic to fail in the region between the pin holes.
U.S. Pat. No. 3,735,466 describes a machine for connecting pins to a ceramic substrate that employs multiple individual bushings attached to a flexible pad which provides an elastic foundation of known spring constant. A heading ram is lowered into contact with the pins to form heads but the substrate is in direct contact with the flexible bushings which move parallel to the applied force and conform to the surface of the substrate, thereby supporting the substrate and reducing the magnitude of bending moments in the substrate that otherwise would cause cracking. A similar method described in the publication by R. J. Modlo et al, Low-stress pin insertion, IBM Technical Disclosure Bulletin, volume 22, number 8B, (January 1980) pages 3649-3650, eliminates bending stresses in the ceramic substrate and reduces the occurrence of cracking of the substrate by employing segmented floating rams and anvils that provide an equal load to each loading point. An equal reaction force is located directly opposite the load point in order to assure that the load applied to fix the pin to the substrate is equal at each ram. A flexible bladder is filled with an incompressible fluid and contacts the top of each pinning ram. Similarly the anvils are supported on a single fluid-filled bladder beneath the substrate.
All of the prior art systems tend to produce an undesirable space or void between the substrate hole and the pin. This void weakens the effectiveness of the interconnection between the pin and the substrate and becomes a repository for undesirable contaminants, which can adversely affect the mechanical connection between the pin and substrate and the electrical conduction properties of the pin.